1. Field of the Invention
The present invention relates to a ceramic multilayer wiring board.
2. Description of Related Art
Japanese Examined Patent Publication (Kokoku) No. Hei-03-78798 discloses a ceramic multi-layer wiring board in which a copper-based material excellent in resistance to migration is used as a surface wiring conductor and a silver-based material resistant to a high firing temperature in an oxygen atmosphere is used as an inner wiring conductor and through-hole conductor. The Kokokyu also discloses covering the surface of a top through-hole conductor with an eutectic-preventing layer before disposing the surface wiring conductor. The eutectic crystal-preventing layer is provided, by CVD or other means, in order to prevent the formation of Ag--Cu eutectic crystals at the interface between the through-hole conductors and the surface wiring conductor.
This method of preventing eutectic crystals necessitates an additional step of forming the eutectic crystal-preventing layer. If the eutectic crystal-preventing layer is formed by CVD, a long time is required to obtain a necessary thickness of the layer. If the layer is formed by a printing method, a firing step is required and a long time is necessary. If the eutectic crystal-preventing layer is thin, silver or copper diffuses through the eutectic crystal-preventing layer and a Ag--Cu eutectic crystal layer is formed.
Japanese Unexamined Patent Publication (Kokai) No. Hei-04-32297 discloses a second surface wiring conductor disposed between the copper surface wiring conductor and the top through-hole conductor, in order to prevent the formation of the Ag--Cu eutectic crystals. The second surface wiring conductor comprises copper and is fired at a temperature (600 .degree. C.) lower than the eutectic temperature of 780.degree. C.
The disposition of the low temperature firing copper surface conductor requires an additional step. The copper surface conductor fired at a low temperature has a low adhesion with the substrate and has a poor wettability with a solder. Thus, the reliability is low.
The object of the present invention is to provide a ceramic multi-layer wiring board comprising a copper-based surface conductor excellent in resistance to migration and a non-copper-based inner wiring layer capable of being fired in an oxidizing atmosphere, by which formation of eutectic crystals between the above two materials is prevented and in which an additional step is not necessary.